The presence of various deoxyribonucleases (hereinafter referred to as DNase) in mammalian cells has been known. DNase II is one of the DNases studied most and catalyzes DNA hydrolysis reaction in the absence of divalent cations at acidic pH [in The Enzymes (Boyer, P. D., ed) 3rd Ed., Vol. 4, pp. 271-287 (1971), Academic Press, New York; Arch. Biochem. Biophys., 300: 440-450 (1993)]. While the acid DNase activities are widely found in various animal tissues [Biochim. Biophys. Acta, 1119: 185-193 (1992); J. Biol. Chem., 273: 2610-2616 (1998)], DNase II has been considered to be the sole enzyme responsible for the acid DNase activities. Because DNase II shows low organ specificity and is distributed ubiquitously, a possibility of DNase II playing an important biological role in the fundamental biological phenomena, such as DNA catabolism and apoptosis, has been suggested [The Enzymes (1971), supra; Arch. Biochem. Biophys., 300: 440-450 (1993)].
Even though the enzymological properties of the DNase II isolated from different organisms are very similar, their physicochemical properties and molecular structures are strikingly different. For example, it is known that porcine DNase II is a complex protein consisting of unidentical subunits derived from its precursor protein, but DNase II derived from other animals are mostly single polypeptides [J. Biol. Chem., 260: 10708-10713 (1985); Biochem. Biophys. Res. Commun., 247: 864-869 (1998); J. Biol. Chem., 251: 116-123 (1976); Gene, 215: 281-289 (1998)]. Furthermore, the apparent molecular weights of DNase II vary from 26.5 kDa to 45 kDa [J. Biol. Chem. (1976), supra; Gene, (1998), supra; J. Biol. Chem., 247: 1424-1432 (1972); Eur. J. Biochem., 202: 479-484 (1991)].
The diversity of acid DNases can be also appreciated from the subcellular localization. DNase II is considered to be localized in lysosomes [J. Biol. Chem. (1972), supra; Biochim. Biophys. Acta, 1007: 15-22 (1989)], but acid DNase activity is also found in nuclear fraction [Arch. Biochem. Biophys. (1993), supra; Biochem. J., 136: 83-87 (1973)].
The reason for such molecular diversity of DNase II still remains unclear, but the aforementioned findings suggest the existence of a different acid DNase distinguishable from DNase II. In fact, the present inventors have identified and partially purified novel acid DNases (DNase α and DNase β) from the nuclear fraction of rat thymus (JP 8-187079 A). In view of the foregoing situation it is considered to be critical for the elucidation of the diversity of acid DNases to search other novel acid DNases and determine their characteristics.
In addition, DNase has been actively studied with the aim of applying same for the prophylaxis and treatment of various diseases. One of the clinical applications of DNase, which has been drawing particular attention in recent years, is an application to the treatment of cystic fibrosis (hereinafter sometimes to be also referred to as CF) [Annu. Rev. Pharmacol. Toxicol., 35: 257-276 (1995); Chest, 107: 65-70 (1995)]. CF is a lethal hereditary disease caused by abnormal chloride ion channel of exocrine glands. In the Caucasian population, one in 2500 newborns suffers from this disease and one in 25 Caucasians is a carrier. About 90% of the CF patients die of respiratory insufficiency caused by intractable infection with Pseudomonas aeruginosa in the inferior airway in their 20's and 30's [Curr. Opin. Pulm. Med., 6: 425-434 (1995)]. Phlegm that is accumulated in the airway to impair the respiratory function is caused by high concentration DNA released from the disrupted leukocytes infiltrating into the inflammatory site. Genentech, Inc. U.S. is selling a recombinant DNase I as a therapeutic agent for CF in Europe and America, which aims at removing the high molecular weight DNA accumulated in the lung, recovering the respiratory function and preventing infectious diseases [Annu. Rev. Pharmacol. Toxicol. (1995), supra; Chest (1995), supra]. DNase I not only degrades DNA, but also depolymerizes F-actin which is abundant in the sputum of CF patients. However, since the resulting monomeric G-actin strongly inhibits DNase I, DNase I is immediately inactivated. Actually, DNase I hardly shows any therapeutic effect. Some attempts have been made to produce a G-actin nonsensitive DNase I by genetic recombination, but satisfactory DNase has not been obtained yet [Proc. Natl. Acad. Sci. USA, 93: 8225-8229 (1996); J. Biol. Chem., 273: 18374-18381 (1998)]. Thus, there is a demand on the identification of a novel G-actin nonsensitive DNase effective for the treatment of CF.
A second interest in the clinical application of DNase is that for the prophylaxis and treatment of infectious diseases. Some DNases are considered to play an important role in the biological defense mechanisms against infection with bacteria and viruses, based on degradation of foreign genomic DNAs. Accordingly, identification of the DNase involved in the prevention of infection in mammals, such as human, and utilization thereof as a medicament are expected to open a new possibility in the prophylaxis and treatment of infectious diseases.
It is therefore an object of the present invention to provide a novel acid DNase and clarify the characteristics of the enzyme, thereby providing critical information for the study of the molecular diversity of acid DNases. It is another object of the present invention to provide a novel G-actin nonsensitive DNase that can be effectively used as a therapeutic agent of CF. It is yet another object of the present invention to provide a novel DNase useful for the prophylaxis and treatment of infectious diseases.